Molecular oxygen enhances H2O2 utilization for the photocatalytic conversion of methane to liquid-phase oxygenates

H2O2 is widely used as an oxidant for photocatalytic methane conversion to value-added chemicals over oxide-based photocatalysts under mild conditions, but suffers from low utilization efficiencies. Herein, we report that O2 is an efficient molecular additive to enhance the utilization efficiency of H2O2 by suppressing H2O2 adsorption on oxides and consequent photogenerated holes-mediated H2O2 dissociation into O2. In photocatalytic methane conversion over an anatase TiO2 nanocrystals predominantly enclosed by the {001} facets (denoted as TiO2{001})-C3N4 composite photocatalyst at room temperature and ambient pressure, O2 additive significantly enhances the utilization efficiency of H2O2 up to 93.3%, giving formic acid and liquid-phase oxygenates selectivities respectively of 69.8% and 97% and a formic acid yield of 486 μmolHCOOH·gcatalyst−1·h−1. Efficient charge separation within TiO2{001}-C3N4 heterojunctions, photogenerated holes-mediated activation of CH4 into ·CH3 radicals on TiO2{001} and photogenerated electrons-mediated activation of H2O2 into ·OOH radicals on C3N4, and preferential dissociative adsorption of methanol on TiO2{001} are responsible for the active and selective photocatalytic conversion of methane to formic acid over TiO2{001}-C3N4 composite photocatalyst.

This research uses molecular oxygen to improve H2O2 utilization efficiency for photocatalytic CH4 oxidation. The authors did provide many evidences to establish a plausible reaction mechanism. However, it is not enough to be accepted by NC, I suggest reject it. Several points have to be addressed to make it a complete work.
Author reply: We appreciate the reviewer's insightful comments very much. In our submission, we report for the first time O2-suppressed H2O2 adsorption on oxides and consequent photogenerated holes-mediated H2O2 dissociation into O2, which provides a promising new strategy to achieve high H2O2 utilization efficiency and excellent photocatalytic performance for photocatalytic oxidation reactions over oxide-based photocatalysts via co-use of H2O2 and O2. In the photocatalytic aqueousphase methane conversion over an optimized TiO2{001})-C3N4 composite photocatalyst during our study, O2 additive significantly enhances the utilization efficiency of H2O2 unprecedentedly up to 93.3%, giving formic acid and liquid-phase oxygenates selectivity respectively of 69.8% and 97% and formic acid yield of 486 μmolHCOOH·gcatalyst -1 ·h -1 . Since H2O2 production is well known as an environment-unfriendly and economic-costly process, in addition to excellent photocatalytic performance, high H2O2 utilization efficiency is also very desirable in photocatalytic selective conversion of methane over oxide-based photocatalysts widely using H2O2. Therefore, we believe that our results truly represent a breakthrough and will attract great attention and exert profound influences. Meanwhile, we have seriously considered the reviewer's comments and revised the manuscript accordingly. We hope that the revised manuscript will be suitable for the publication in Nature Communications.
1 As photocatalyst, its photophysical and photoelectrical properties before and after synthesis should be studied, such as UV-vis diffuse reflectance spectra, band structures, photo-response curves, electrochemical impedance spectroscopies. The authors should give more discussions in main text.
2 In photocatalytic CH4 conversion, are all products formed at the same time? Or in particular order? It is suggested to study the change of product concentration over reaction time.
Author reply: We appreciate the reviewer's insightful comments very much. Following the reviewer's comments, we examined initial evolutions of reaction products as a function of reaction time over TiO2{001}-C3N4-0.1. The results have been added in the revised manuscript as the following: "Initial evolutions of reaction products as a function of reaction time were examined over TiO2{001}-C3N4-0.1 (Supplementary Table 11). At a reaction time of 10 min, CH3OOH, CH3OH and HCHO were detected, and CH3OOH was the major product. The CH3OOH, CH3OH and HCHO productions increased at a reaction time of 30 min, meanwhile, HCOOH and CH3CH2OH appeared. As a reaction time of 1 h, the CH3OOH production decreased and HCHO was not detected, whereas the CH3OH and HCOOH productions increased greatly and the CH3CH2OH production increased slightly, meanwhile, CH3COOH emerged. These observations suggest CH3OOH as the primary product and CH3OH, HCHO, HCOOH, CH3CH2OH and CH3COOH as the secondary products that are produced sequentially. Moreover, the reaction rate of HCHO seems to be faster than the formation rate." (please see  3 How about the light and chemical stabilities of catalyst after being used many times? And catalytic effect?
Author reply: We appreciate the reviewer's insightful comments very much. Following the reviewer's comments, we evaluated the stability of TiO2{001}-C3N4-0.1 photocatalyst and found that TiO2{001}-C3N4-0.1 is stable and its performance maintains well within six cycles of photocatalytic activity evaluations. Routine structural characterization results, including XPS, XPS, UV-Vis spectra and photocurrent measurements, show few difference between the as-synthesized and used TiO2{001}-C3N4-0.1 catalysts.by initial evolutions of reaction products as a function of reaction time over. The results have been added in the revised manuscript as the following: "TiO2{001}-C3N4-0.1 is stable and its performance maintains well within six cycles of photocatalytic activity evaluations ( Supplementary Fig. 4). Routine structural characterization results ( Supplementary Fig. 5), including XPS, VB-XPS, UV-Vis spectra and photocurrent measurements, show few difference between the as-synthesized and used TiO2{001}-C3N4-0.1 catalysts." (please see  prove that the oxygen in liquid products mainly comes from H2O2, rather than oxygen and water, but it cannot be inferred that oxygen promotes photocatalytic CH4 conversion.
Author reply: We appreciate the reviewer's insightful comments very much. We have rewritten the commented sentence more rigorously in the revised manuscript as the following: "Therefore, during photocatalytic aqueous-phase CH4 conversion in the presence of H2O2 and O2, CH4 preferentially reacts with H2O2 to produce liquid-phase oxygenates, while O2 acts mainly as a promoter to enhance H2O2 utilization efficiency and consequently CH4 conversion, and minorly as a reactant." (please see Lines 207-210) 5 Please add the decomposition rate of H2O2 into O2 by catalysts in the presence of H2O2, catalyst and/or light.
Author reply: We appreciate the reviewer's kind suggestions very much. We have added the decomposition rate of H2O2 under different conditions in Fig. 1e and Supplementary Tables 2 and 3, and described the results in the revised manuscript as the following: "As shown in Fig. 1e, the H2O2 decomposition percentage/H2O2 decomposition rate/O2 selectivity are 31.2%/610.9 μmol·h -1 /93.0% over TiO2{001} NCs in the Ar atmosphere and decrease to 15.4%/301.5 μmol·h -1 /91.8% in the 10% O2/Ar atmosphere, while they are 20.4%/399.4 μmol·h -1 /89.0% over TiO2{001}-C3N4-0.1 in the Ar atmosphere and decrease to 8.26%/161.7 μmol·h -1 /86.4% in the 10% O2/Ar atmosphere." (please see Lines 88-93) We appreciate the reviewer's effort on reviewing our manuscript very much, and we hope that the revised manuscript will be suitable for the publication in Nature Communications.
In this manuscript, the author reported that the introduction of O2 could enhance the utilization efficiency of H2O2 by suppressing H2O2 adsorption on oxides and consequent photogenerated holesmediated H2O2 dissociation into O2. Detailed characterizations and DFT study were carried out to explain the effect of Z-scheme TiO2-C3N4 heterojunctions to photocatalytic methane oxidation with H2O2. However, there are several crucial issues to be solved.
Author reply: We appreciate the reviewer's positive recommendation and insightful comments very much. We have seriously considered the reviewer's comments and revised the manuscript accordingly. We hope that the revised manuscript will be suitable for the publication in Nature Communications.
1. The lattice fringe should be re-measured and lattice spacing in TEM images must be calculated again because the yellow lines in TEM images were misplaced and ambiguous. The lattice spacing of C3N4 should be provided to explain the successful synthesis for heterojunction catalysts (Zscheme TiO2-C3N4).
Author reply: We appreciate the reviewer's insightful comments very much. We have re-measured lattice fringes and clearly marked the values and assigned crystal planes in HRTEM images shown in Supplementary Figs. 1 and 3. We are sure that the lattice fringes of 0.24 and 0.35 nm arise from anatase TiO2 {001} and {101} crystal planes, respectively. We also re-took the HRTEM and elementary mapping images of our samples with higher qualities. Unfortunately, we failed to observe clear lattice fringes of g-C3N4 in the HRTEM images ( Supplementary Fig. 3d), which is rather well know due to the strong damage effect of high-energy electron beam on the structure of g-C3N4, but its presence in the TiO2 NCs-C3N4 composites is identified by XRD patterns (Supplementary Fig. 3e) and XPS spectra ( Supplementary Fig. 3f).
In the revised manuscript, we have included a separate paragraph to describe synthesis and routine spectroscopic and microscopic characterization results of our TiO2 NCs and TiO2 NCs-C3N4 composite photocatalysts as the following: "Synthesis and structural characterizations. Anatase TiO2 nanocrystals (NCs) predominantly enclosed by the {001} facets (denoted as TiO2{001}), the {100} facets (denoted as TiO2{100}) and the {101} facets (denoted as TiO2{101}) were prepared following well-established recipes 27 . XRD patterns, TEM and HRETM images of as-synthesized various TiO2 NCs (Fig. 1a, Supplementary  Fig. 1) agree with those reported previously 27 . TiO2 NCs-C3N4 composites were prepared by calcination of mixture of calculated amounts of dicyandiamide (C₂H₄N₄) and TiO2 NCs in Ar at 550 ℃ and denoted as TiO2 NCs-C3N4-x, in which x was the actual TiO2:C3N4 mole ratio acquired by TGA analysis (Supplementary Fig. 2 and Table 1). TEM, HRTEM and element mapping images (Figs. 1 b-d, Supplementary Fig. 3 a-c) show that various TiO2 NCs preserve their original morphologies and form smooth anatase TiO2-g-C3N4 interfaces. We failed to observe clear lattice fringes of g-C3N4 in the HRTEM images ( Supplementary Fig. 3d) likely due to the strong damage effect of highenergy electron beam on the structure of g-C3N4, but its presence in the TiO2 NCs-C3N4 composites is identified by XRD patterns (Supplementary Fig. 3e) and XPS spectra ( Supplementary Fig. 3f)." (please see Lines 62-75) 2. The light wavelength of 300 W Xe lamp should be added. Blank experiments must be made if the wavelength of light illumination was UV range, for example, no catalysts and no CH4 with TiO2{001}-C3N4 catalyst to exclude the carbon source on C3N4 support.

Author reply:
We appreciate the reviewer's kind suggestions very much. We have added the spectrum of used Xe light in the revised Supplementary Information (please see Page S3).
We also did blank photocatalytic experiment of photocatalytic reaction in the presence of TiO2{001}-C3N4-0.1 but absence of CH4 in the reactant and did not detected any C-contained products. The results, together with the 13 CH4 experimental results, demonstrate that all C-contained products exclusively form from CH4. We have described these results in the revised manuscript as the following: "Blank photocatalytic experiment of photocatalytic reaction in the presence of TiO2{001}-C3N4-0.1 but absence of CH4 in the reactant did not produce detectable C-contained products; meanwhile, using 13 CH4, all C-contained products only contained 13 C ( Supplementary Fig. 6). Thus, all Ccontained products exclusively form from CH4." (please see Lines 142-145) 3. The authors only showed the selectivity of various oxygenates, but the yield or productivity of all products should be exhibited especially in the manuscript to compare the change of photocatalytic activity after adding O2.
Author reply: We appreciate the reviewer's kind suggestions very much. We have added the yields of all products in Fig. 1 f and g and Supplementary Tables 5-10, and described the results in the revised manuscript as the following: "the HCOOH yield from 12.0 to 37.6 μmol·gcatalyst -1 ·h -1 ," (please see Line 118) "the HCOOH yield from 202.2 to 486 μmol·gcatalyst -1 ·h -1 ," (please see Lines 125 and 126) 4. For products analysis, CH2O species at 1712 cm -1 were detected on in situ DRIFTS spectra, and •CHO radicals were proposed in the reaction paths of TiO2 NCs and TiO2 NCs-C3N4. However, the authors did not quantify the generation of HCHO. According to enormous references on photocatalytic methane oxidation over TiO2 photocatalysts, HCHO can be the prominent overoxidized products. Therefore, it is necessary to provide analysis about HCHO.
Author reply: We appreciate the reviewer's insightful comments very much. Our in situ DRIFTS spectra demonstrate the formation of adsorbed HCHO as a surface intermediate during photocatalytic aqueous-phase CH4 conversion to HCOOH using H2O2 and O2. We analyzed all likely products but did not detect the formation of HCHO under the studied reaction conditions. As suggested by the reviewer 1, we examined initial evolutions of reaction products as a function of reaction time over TiO2{001}-C3N4-0.1. The HCHO formation appears at a reaction time of 10 min, increases at a reaction time of 30 min, but disappears at a reaction time of 1 h. These observations suggest that the reaction rate of HCHO seems to be faster than the formation rate, which makes HCHO undetectable. We have added these results in the revised manuscript as the following: "Initial evolutions of reaction products as a function of reaction time were examined over TiO2{001}-C3N4-0.1 (Supplementary Table 11). At a reaction time of 10 min, CH3OOH, CH3OH and HCHO were detected, and CH3OOH was the major product. The CH3OOH, CH3OH and HCHO productions increased at a reaction time of 30 min, meanwhile, HCOOH and CH3CH2OH appeared.
As a reaction time of 1 h, the CH3OOH production decreased and HCHO was not detected, whereas the CH3OH and HCOOH productions increased greatly and the CH3CH2OH production increased slightly, meanwhile, CH3COOH emerged. These observations suggest CH3OOH as the primary product and CH3OH, HCHO, HCOOH, CH3CH2OH and CH3COOH as the secondary products that are produced sequentially. Moreover, the reaction rate of HCHO seems to be faster than the formation rate." (please see Lines 145-155) 5. We disagreed that "O2 mainly acts as a promoter for photocatalytic CH4 conversion with H2O2, instead of as a reactant" at line 154. Firstly, •O2radicals were detected by in situ ESR spectra, it is easy to form •OOH by combination •O2with H + . Secondly, the authors presented O2 participated in the generation of CH3OOH in Figure 2. Finally, CH3 18 OH, HCO 18 OH, CH3HC2 18 OH and CH3CO 18 OH were detected by mass spectra when using 18 O2. So the above conclusions is contradictory with "O2 mainly acts as a promoter for photocatalytic CH4 conversion with H2O2, instead of as a reactant".
Author reply: We appreciate the reviewer's insightful comments very much. We have rewritten the commented sentence more rigorously in the revised manuscript as the following: "Therefore, during photocatalytic aqueous-phase CH4 conversion in the presence of H2O2 and O2, CH4 preferentially reacts with H2O2 to produce liquid-phase oxygenates, while O2 acts mainly as a promoter to enhance H2O2 utilization efficiency and consequently CH4 conversion, and minorly as a reactant." (please see Lines 207-210) 6. If authors wanted to obtain the conclusion about "photocatalytic CH4 activation to •CH3 radicals is mainly mediated by h + "in line 179, the "DMPO-CH3" signals between the in situ ESR spectra of CH4 and that of CH4+isopropyl alcohol mixture in Figure S12 should be compared under the same level. The author should provide in situ ESR spectra of CH4+hole trapping agents to determine the oxidation of methane to •CH3 is mainly by h + .
Author reply: We appreciate the reviewer's insightful comments very much. We have compared the in situ ESR spectra of CH4+H2O and CH4+ isopropyl alcohol+H2O under UV light illumination in the presence of DMPO over TiO2 NCs and TiO2 NCs-C3N4-0.1 composites at 298 K in Supplementary Fig. 17 of revised manuscript. CH3 radicals greatly grew when isopropanol was added to quench ·OH radicals, supporting that the formation of CH3 radicals is not related to ·OH radicals. This also suggests the likely reaction between co-existingCH3 and ·OH radicals. Meanwhile, CH3 radicals were not observed in the in situ ESR spectra of CH4+methanol mixture (CH4 +5 mL CH3OH+3µL DMPO) + H2O2 + O2 under UV light illumination in the presence of DMPO over TiO2 NCs and TiO2 NCs-C3N4-0.1 composites at 298 K ( Supplementary Fig. 18) due to the quench of h + by methanol, but O2radicals appear. Thus, photocatalytic CH4 activation to CH3 radicals is mainly mediated by h + , instead of byOOH, OH and O2radicals.
The relevant results are described in the revised manuscript as the following: "When CH4 was introduced to the aqueous solutions containing TiO2 NCs or TiO2 NCs-C3N4-0.1 composites under UV light illumination (Fig. 2f), CH3 radicals 22,26 , in addition to OH radicals, were detected. They greatly grew when isopropanol was added to quench ·OH radicals ( Supplementary Fig. 17), but could not be detected in the presence of H2O2 and O2 when h + was quenched using methanol ( Supplementary Fig. 18). Thus, photocatalytic CH4 activation to CH3 radicals is mainly mediated by h + , instead of byOOH, OH and O2radicals." (please see Lines 227-233) 7. The author should recheck the assignments of vibrational bands observed in the in situ DRIFTS spectra especially about carbonates (1504 and 1592 cm -1 ) in methane oxidation reaction. Meanwhile, in situ DRIFTS spectra among the 2100-2400 cm -1 should be given to exclude the generation of COx.
Author reply: We appreciate the reviewer's insightful comments very much. As discussed in our manuscript, the observed vibrational bands (Supplementary Table 14) were assigned based on in situ DRIFTS spectra of CH3OH and HCOOH adsorption on various TiO2 NCs (Supplementary Fig.  28) and previous reports 34-36 (please see Lines 355-358). The assignments of carbonates on TiO2 have been ambiguous due to the presence of different types of carbonates. According to our own work [Ref. 34, J. Phys. Chem. C 120, 21472-21485 (2016) and ACS Catal. 12, 6457-6463 (2022)], the peak at 1592 cm -1 can be assigned to bidentate carbonate while that at 1504 cm -1 can be assigned to monodentate carbonate, We believe that our assignments are reliable.
In the revised manuscript, we have added the papers J. Phys. Chem. C 120, 21472-21485 (2016) and ACS Catal. 12, 6457-6463 (2022) as Ref. 35 and 36 to further support our assignments. We have also re-ordered all references accordingly.
Meanwhile, we have included the in situ DRIFTS spectra among the 2100-2400 cm -1 to Figure 4a in the revised manuscript, in which no COx signal could be seen.
8. The photocatalytic reaction path of CH4 photocatalytic in Figure 2g and 2h was inadequate. For example, the generation of •O2radicals should be represented on the schemes. The author proposed the formation of CH3OH was mainly the reduction of CH3OOH, but it was easy to form CH3OH by the coupling of •CH3 and •OH. In addition, authors did not explain the oxidation pathway of HCOOH from •OH radicals or holes.
Author reply: We appreciate the reviewer's insightful comments very much. It is unlikely to schematically show the while reaction network of our photocatalytic reactions in Fig. 2 g and h due to the complexity. The contents of Fig. 2 g and h are to schematically show the proposed dominant photocatalytic aqueous-phase CH4 reaction paths to liquid-phase oxygenates in the presence of H2O2 and O2 over TiO2 NCs and TiO2 NCs-C3N4 based on the photocatalytic reaction data, ESR results, isotope-labelling results, in situ DRIFTS results, and the literature's reports. The description and discussion of Fig. 2 g and h can be found at [259][260][261][262][263][362][363][364][365] In the revised manuscript, we have re-written the caption of Fig. 2 g and h to clarify their contents as the following: "Schematic diagrams of proposed dominant photocatalytic aqueous-phase CH4 reaction paths to liquid-phase oxygenates in the presence of H2O2 and O2 over (g) TiO2 NCs and (h) TiO2 NCs-C3N4." (please see Lines 169 and 170) This paper reported the photocatalytic methane (CH4) conversion by H2O2 as the oxidant over TiO2{001})-C3N4 composite photocatalysts, in which, interestingly, O2 is used a molecular additive to enhance the utilization efficiency of H2O2 up to 93.3%, and the selectivity of formic acid and liquid-phase oxygenates is up to 69.8% and 97%, respectively. According to the studies of the NMR, ESR, and in-situ FTIR techniques, the photocatalytic mechanism of CH4 is proposed. Thus, I can recommend the publication of this manuscript after a major revision: Author reply: We appreciate the reviewer's positive recommendation and insightful comments very much. We have seriously considered the reviewer's comments and revised the manuscript accordingly. We hope that the revised manuscript will be suitable for the publication in Nature Communications.
1. An interesting discovery that O2 can enhance the utilization efficiency of H2O2 effectively was proposed, which, however, was not discussed comprehensively how O2 can improve the utilization efficiency of H2O2. It was found that the oxygen atoms in CH3OOH, CH3OH, HCOOH and CH3CH2OH are contributed majorly by H2O2 and minor by O2, but seldom by H2O, while the oxygen atoms of CH3COOH are contributed majorly by H2O, and minor by O2. (Figure 2) As such, the authors proposed that O2 mainly acts as a promoter for photocatalytic CH4 conversion with H2O2, instead of as a reactant. (Page 5, line 155) This expression is inappropriate, because O2 is clearly involved in the reaction, and the oxygen atom of O2 transfers into all the products. The low proportion of products from O2 oxidation may be due to the low amount of O2. Therefore, it is necessary to detect and discuss the activity and selectivity of photocatalytic CH4 conversion by H2O2 with the variable amount of O2.
Author reply: We appreciate the reviewer's insightful comments very much. Following the reviewer's comments, we the O2 concentration in the reactant was increased from 4% (8%CH4+4%O2+88%Ar+165μL H2O2+20mL H2O) to 12% (8%CH4+12%O2+80%Ar+165μL H2O2+20mL H2O), and the photocatalytic reaction was studied over TiO2{001}-C3N4-0.1 comparatively with 16 O2 or 18 O2 in order to further clarify the role of O2. The results show that using 16 O2 or 18 O2 gave similar H2O2 utilization efficiencies of around 94% and slightly different CH4 conversion rates and product selectivity. Using 18 O2, the CH3 18 OH/CH3 16 OH, HC 16 O 18 OH/HC 16 O 16 OH and CH3CH2 18 OH/CH3CH2 16 OH ratios in the liquid-phase products were measured respectively as around 0.19, 0.17 and 0.22, similar to the case of the reactant with 4% O2; however, C 18 O and C 18 O2 were detected and the fraction of C 16 O 18 O in CO2 is much larger than that of C 16 O 16 O, different from the case of the reactant with 4% O2. These results prove that, during photocatalytic aqueous-phase CH4 conversion in the presence of H2O2 and O2, CH4 preferentially reacts with H2O2 to produce liquid-phase oxygenates, while O2 acts mainly as a promoter to enhance H2O2 utilization efficiency and consequently CH4 conversion, and minorly as a reactant.
We have included these results in the revised manuscript as the following: "In order to further clarify the role of O2, the O2 concentration in the reactant was increased from 4% (8%CH4+4%O2+88%Ar+165μL H2O2+20mL H2O) to 12% (8%CH4+12%O2+80%Ar+165μL H2O2+20mL H2O), and the photocatalytic reaction was studied over TiO2{001}-C3N4-0.1 comparatively with 16 O2 or 18 O2. Using 16 O2 or 18 O2 gave similar H2O2 utilization efficiencies of around 94% and slightly different CH4 conversion rates and product selectivity (Supplementary  , similar to the case of the reactant with 4% O2; however, C 18 O and C 18 O2 were detected and the fraction of C 16 O 18 O in CO2 is much larger than that of C 16 O 16 O, different from the case of the reactant with 4% O2. Therefore, during photocatalytic aqueous-phase CH4 conversion in the presence of H2O2 and O2, CH4 preferentially reacts with H2O2 to produce liquid-phase oxygenates, while O2 acts mainly as a promoter to enhance H2O2 utilization efficiency and consequently CH4 conversion, and minorly as a reactant." (please see  2. According to the previous reports, HCHO should be formed in the photocatalytic reaction over TiO2 photocatalysts. It should be discussed why the HCHO is not produced in the CH4 oxidation herein.
Author reply: We appreciate the reviewer's insightful comments very much. We analyzed all likely products but did not detect the formation of HCHO under the studied reaction conditions. As suggested by the reviewer 1, we examined initial evolutions of reaction products as a function of reaction time over TiO2{001}-C3N4-0.1. The HCHO formation appears at a reaction time of 10 min, increases at a reaction time of 30 min, but disappears at a reaction time of 1 h. These observations suggest that the reaction rate of HCHO seems to be faster than the formation rate, which makes HCHO undetectable. We have added these results in the revised manuscript as the following: "Initial evolutions of reaction products as a function of reaction time were examined over TiO2{001}-C3N4-0.1 (Supplementary Table 11). At a reaction time of 10 min, CH3OOH, CH3OH and HCHO were detected, and CH3OOH was the major product. The CH3OOH, CH3OH and HCHO productions increased at a reaction time of 30 min, meanwhile, HCOOH and CH3CH2OH appeared. As a reaction time of 1 h, the CH3OOH production decreased and HCHO was not detected, whereas the CH3OH and HCOOH productions increased greatly and the CH3CH2OH production increased slightly, meanwhile, CH3COOH emerged. These observations suggest CH3OOH as the primary product and CH3OH, HCHO, HCOOH, CH3CH2OH and CH3COOH as the secondary products that are produced sequentially. Moreover, the reaction rate of HCHO seems to be faster than the formation rate." (please see Lines 145-155)